This invention relates to a semiconductor device in which an N type impurity is diffused in a P type base layer of an NPN type transistor of vertical structure, and to a method of manufacturing this device.
In the past, N type silicon wafers have been used to make NPN type transistors of vertical structure. First of all, a boron doped silicate glass (BSG) layer is deposited on the exposed part of an N type silicon wafer provided with an opening in silicon oxide, and then a chemical vapor deposition (CVD) layer is deposited on this BSG layer. This silicon wafer is heat treated, whereby the boron doped in this BSG layer is diffused in the N type silicon wafer and a P type base layer is formed. Next, a coating of silicon oxide is made to adhere to the whole surface of this N type silicon wafer, and in this silicon oxide layer the silicon oxide layer covering the P type base layer is removed. After that, this wafer is subjected to heat treatment in a gaseous mixture of phosphorus (P) and arsenic (As) and an N type emitter layer is formed in the P type base layer. After this, electrodes are formed in the P type base layer and the N type emitter layer of the N type silicon wafer and this N type silicon wafer is divided and silicon pellets are obtained. When this prior art NPN type transistor is made, an emitter dip region is formed in the base region of this NPN type transistor, that is to say the emitter dip phenomenon appears. The emitter dip region 5 is shown in FIG. 1.
When phosphorus (P) and arsenic (As) are diffused from the exposed part of the N type silicon wafer 2 in which the P type base layer 3 has been formed and from which silicon oxide has been removed, so that the N type emitter layer 4 is formed in the P type base layer 3, the vacancies V which are a defect present in the surface of the N type silicon wafer become (P - V.sup.=) pairs with the phosphorus (P), and these (P - V.sup.=) and phosphorus (P) are diffused in the direction of the depth of the N type silicon wafer in which the phosphorus (P) is diffused. Near the boundary of the P type base layer 3 in which the phosphorus concentration is low and the emitter layer 4, these (P - V.sup.=) pairs dissociate into (P.sup.+ - V.degree.) pairs, and these further dissociate into P.sup.30 and V.degree.. When these dissociated vacancies V.sup.o increase in the vicinity of the boundary of the P type base layer 3 and the N type emitter layer 4, the coefficient of diffusion of the boron (B) of the P type base layer 3 in the neighborhood of this boundary, that is to say in the tail region, becomes great and the emitter dip region 5 is formed in the base region of the transistor. Electric field concentration readily occurs in this emitter dip region, and the voltage resistance of this NPN type transistor is impaired. Also, numerous vacancies V present in the base layer and the vicinity of the emitter dip region become recombination centers, and the "fluctuation effect" occurs whereby the current flowing in this NPN transistor constantly fluctuates, and this therefore became a cause of noise in this semiconductor device.